1. Field of the Invention
The present invention relates to a carrier for electrophotography, a process for producing the carrier, a two-component type developer having the carrier and a toner, and an image forming method.
2. Related Background Art
A variety of methods are known for electrophotography, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth. In these methods, a photoconductive layer is imagewise exposed to light, corresponding to an original to form thereon an electrostatic image. Then, in the case of normal development, a toner having a polarity opposite to that of the electrostatic image is caused to adhere thereto to develop the electrostatic latent image. Next, the toner image formed is transferred to a transfer medium such as paper if necessary, followed by fixing by the action of heat, pressure, heat-and-pressure or solvent vapor. Thus, a copy is obtained.
In the step of developing the electrostatic image, an electrostatic mutual action between charged toner particles and the electrostatic image is utilized to form the toner image on the electrostatic image. In general, among methods of developing such electrostatic images by the use of toners, two-component type developers prepared by blending toner particles and carrier particles are preferably used in full-color copying machines required to achieve an especially high image quality.
The carrier particles that constitute the two-component type developers can be roughly grouped into conductive carriers and insulative carriers. The conductive carriers are usually comprised of oxidized or unoxidized iron powder. Two-component type developers comprised of such iron powder have had the problems that their triboelectric chargeability to toner is unstable, and charges on a photosensitive drum may leak because of the use of conductive carriers to cause a lowering of image quality, or carrier adhesion may occur because of charges injected from the conductive carrier into a photosensitive member, to cause carrier adhesion at non-image areas. Such problems especially occur especially when carrier cores are made to have a lower magnetic force in order to obtain copy images with a high image quality and a high vividness, which also cause a lowering of image quality, and hence it has been unsuitable for the conductive carriers to be used in electrophotographic processes for forming copy images with a high image quality and a high vividness.
The insulative carriers are commonly typified by a resin-coated carrier comprising carrier core particles comprised of a ferromagnetic material such as iron, nickel or ferrite, or magnetic material disperse type resin cope particles prepared by dispersing magnetic fine particles in a resin, and whose surfaces are coated with an insulating resin.
It is true that as disclosed in Japanese Patent Application Laid-open No. 58-21750 the coating of core particles brings about an improvement in longevity properties, impact resistance, resistance values and breakdown resistance to applied voltage, but it is very difficult to bring the resistivity of carriers to a proper value and also to uniformly control the state of coating.
In the case of the magnetic material disperse type resin carriers, faulty coating may cause fall-off of magnetic fine particles from carrier particles surfaces, and also may cause partial charge-up of carriers, bringing about the problem that, especially in a developing system of applying an alternating electric field in order to make image quality higher, its electrostatic force tends to cause carrier adhesion.
In the developing process where a high-frequency alternating electric field is applied, as required especially in high-speed electrophotographic copying machines and when images are formed in a high image quality and a high vividness, the above resin-coated insulating carrier may cause carrier charge-up as a result of accumulation of charged components produced on the surfaces when it comes into friction with other carrier particles and toner particles in a developing assembly, to cause a great variation of development efficiency, so that the image density may increase as a result of running or the triboelectric chargeability may become lower to cause in-machine toner scatter. The carrier charge-up may remarkably occur especially in an environment of low temperature and low humidity, often bringing about problems.
As a means for making improvements from such aspects, it is proposed to use a medium-resistance material as a carrier coat agent. It is true that the used of the medium-resistance material as a carrier coat agent brings about an improvement in regard to the problem caused along the phenomenon of charge-up occurring during a high-speed process or in a high-frequency alternating electric field, but such materials have caused problems in that the image quality deteriorates because of the disorder of electrostatic images and the charge injection from developing sleeves into carriers causes the phenomenon of carrier adhesion.
In recent years, with a progress in computers, high-vision systems and so forth, there is a demand for more highly minute full-color image output means. To this end, efforts have been made so that full-color images can have image quality and vividness higher enough to achieve a high quality comparable to the level of image quality of silver salt photographs. In answer to such a demand, studies are made from various directions or perspectives, such as processes, materials and so forth. For example, from the viewpoint of electrophotographic processing, there can be methods of converting the analog processing of images into digital processing, or applying an alternating bias during development to vibrate developing (magnetic) brushes. From the perspective of developers, there is a method of making carrier and toner particle diameters smaller.
Based on detailed studies of electrophotographic processing, there is a possibility that a higher image quality can be achieved by densifying the developing (magnetic) brush on a developing sleeve. The developing brush can be made dense by decreasing the magnetic force of carrier particles used.
It has been hitherto studied to decrease magnetic properties of carriers. For example, Japanese Patent Application Laid-open No. 59-104663 discloses a method in which a magnetic carrier having a small saturation magnetization is used. Although the use of carrier having a small saturation magnetization can bring about an improvement in fine-line reproduction, it, on the other hand, causes a decrease in the force of binding carrier particles onto the developing sleeve, to tend to cause the phenomenon of carrier adhesion where magnetic carrier particles transfer to the photosensitive drum to cause faulty images.
The phenomenon of carrier adhesion is known to tend to occur also because of the use of magnetic carriers with a small particle diameter. For example, Japanese Patent Application Laid-open No. 60-131549 discloses a method in which images are formed using a magnetic carrier and a toner which have been made to comprise fine particles. This publication discloses that, in order to better prevent carrier adhesion in a developing process where a vibrating electric field is applied, it is effective to make carriers have a high resistivity.
However, even if the bulk resistivity of carriers is made higher in order to prevent carrier adhesion, this has been unsatisfactory in some instances in order to better prevent carrier adhesion and achieve a higher image quality.
To obtain coated carriers, various methods are known as disclosed, for example, in Japanese Patent Publication No. 47-20755, Japanese Patent Application Laid-open No. 48-94442, Japanese Patent Publication No. 54-97354, Japanese Patent Applications Laid-open No. 56-97354, No. 56-113146, No. 58-202457 and No. 58-202457, Japanese Patent Publication No. 59-33911, Japanese Patent Applications Laid-open No. 61-149296 and No. 3-140969, etc. However, it has been long sought to provide a developer that can form toner images free of carrier adhesion and with a high image quality.
As discussed above, in order to make image quality higher, it has been long sought to provide a carrier that can solve the above problems.